Thank you Lynx for bringing davidfor's solution out from being buried in all that technical CSS thread, I completely missed this answer to that very annoying problem. It's easy to see how Kobo missed this bug, since before 2.3.1 the margin on sideloaded books was zero regardless of what setting you used. Hopefully they'll fix it in the next firmware.

And big thanks to davidfor again. That's the second time in a week I'm in you debt, after also helping me be finally able to display Hebrew books the right way.

It's easy to see how Kobo missed this bug, since before 2.3.1 the margin on sideloaded books was zero regardless of what setting you used.

I think you're mistaken. I'm still using v2.1.5 on my Glo and the margins slider works fine (the values aren't to my liking but that's a different story).

If you're having trouble with sideloaded epubs it's more likely that you have margins hard-coded into your epub css files, either in the <body> class or perhaps an @page {...} statement. I think the Kobo will honour those settings if they exist.

I think you're mistaken. I'm still using v2.1.5 on my Glo and the margins slider works fine (the values aren't to my liking but that's a different story).

If you're having trouble with sideloaded epubs it's more likely that you have margins hard-coded into your epub css files, either in the <body> class or perhaps an @page {...} statement. I think the Kobo will honour those settings if they exist.

Firmware 2.3.x will modify the margins on an epub that has hard coded margins with one big caveat -- you can only increase the margins from what is hard coded. I posted a couple of versions of Charles Stross' Toast ebook, one with narrow margins and one with wide margins which demonstrated that effect. The narrow margin version always had narrower margins at the same slider setting.

To quote my original post:

On my Glo measuring from the left edge of the screen to the H in "Highly flammable.", I got the following results.

Margin slider fully to the left:
Toast-narrow has a 2mm margin
Toast-wide has a 17mm margin

Margin slider fully to the right:
Toast-narrow has a 16mm margin
Toast-wide has a 31mm wide margin

This is using the font size my tired old eyes find comfortable. At the lowest font size settings, the margins on Toast-wide go from 6mm to 20mm which is about the same 14mm change.

I'm not sure, how the Kobo is applying its page margin setting. Is it overwriting the @page or the body css. Or is it possibly adding to one or the other?

If you want to add top/bottom page margins then the only effective way is to use the @page -- but it doesn't look like the Kobo lets you use the slider to change these. I think you'd have to hard-code it in the epub, either manually or using calibre. Even then I'm not sure whether the Kobo would overwrite the values with zero.

I can't test at the moment as my PC is too flaky to use for long periods.

I'm not sure, how the Kobo is applying its page margin setting. Is it overwriting the @page or the body css. Or is it possibly adding to one or the other?

If you want to add top/bottom page margins then the only effective way is to use the @page -- but it doesn't look like the Kobo lets you use the slider to change these. I think you'd have to hard-code it in the epub, either manually or using calibre. Even then I'm not sure whether the Kobo would overwrite the values with zero.

I can't test at the moment as my PC is too flaky to use for long periods.

As far as I can in some quick testing, the margins are additive. The margin used seems to be the sum of the margins set by the various applicable style elements. That is if the body style is set to 25 points and the paragraph style is set to 5 pts, the paragraph will display with 30 point margins.
Simplified stylesheet for my testing was something like:
.body {margin: 0 25pt;}
.para {margin: 0 15pt;}
.sentence {margin: 0 20pt;}

(the margins set should be top&bottom 0, right and left xxpt)

This gave me a final margin which was visually equivalent to setting the body margin to 60pt and the rest to 0 for that sentence.

The Kobo's margin slider seems to act as just another element. for the left & right margins.

This was using ADE 2.0 to display the epubs -- just a touch faster than connecting a Kobo and waiting through the restart and processing content.